1. Field of the Invention
The present disclosure relates to a structure of a ventilation fan such as a duct fan or a structure of a single suction type centrifugal fan used in an air conditioner or the like.
2. Description of the Related Art
In recent years, a centrifugal fan has been requested to acquire a higher static pressure to cope with a tendency that duct piping connected to the centrifugal fan becomes more complicated or airtightness of a building is increased. As the configuration of a conventional single suction type centrifugal fan which realizes the higher static pressure, there has been known a centrifugal fan having a turbulence suppressing plate which is disposed in a vicinity of a tongue portion and on an inner peripheral portion side of blades of an impeller, and which extends along the radial direction of the impeller.
Hereinafter, such a conventional example is described with reference to FIG. 5A and FIG. 5B.
As shown in FIG. 5A and FIG. 5B, single suction type centrifugal fan 101 is formed of casing 102, and impeller 103 housed in casing 102. Casing 102 is formed of suction-side plate 105 having suction opening 104, vortex-like scroll 106, and motor fixing side plate 108 to which motor 107 is fixed. One end of scroll 106 is connected to suction-side plate 105 and the other end of scroll 106 is connected to motor fixing side plate 108. Casing 102 is formed in a spiral shape such that a flow passage cross-sectional area (radial cross-sectional area in a region surrounded by an outer peripheral side of impeller 103, an inner side of scroll 106, suction-side plate 105, and motor fixing side plate 108) is gradually enlarged along rotational direction 110 of impeller 103 from tongue portion 109.
Impeller 103 is fixedly connected to motor 107. When impeller 103 is rotated by driving motor 107, suction gas flow 111 flows into casing 102 from intake port 104 by way of impeller 103. Air blown off from impeller 103 is boosted in spiral casing 102, a pressure of the air is converted into a static pressure from a dynamic pressure, and the gas flows out from discharge port 112 as discharge gas flow 113.
When a high static pressure is applied to the centrifugal fan due to a tendency that duct piping connected to the centrifugal fan becomes more complicated or airtightness of a building is increased, that is, when the centrifugal fan is used on a shutoff side on performance (PQ: Static Pressure (Pressure)-Air quantity (Quantity)) curve, discharge gas flow 113 does not flow out as a laminar flow. The shutoff side is the point of zero airflow on the static pressure curve. As a result, in the vicinity of tongue portion 109, tongue portion turbulence 114 occurs which flows backward into the inside of impeller 103 from the inside of casing 102. The centrifugal fan boosts a pressure of a gas flow by spiral casing 102 which starts from tongue portion 109. However, in the case of the generally used single suction type centrifugal fan, a gas flow at a pressure boosting start position in the vicinity of tongue portion 109 becomes turbulent due to tongue portion turbulence 114 and hence, pressure boosting start position is retracted. As a result, a pressure is not sufficiently boosted by spiral casing 102 thus giving rise to a drawback that performance (static pressure) on a shut-off side is lowered. To cope with such a drawback, in single suction type centrifugal fan 101 described in Unexamined Japanese Patent Publication No. 8-284894, as shown in FIG. 5A and FIG. 5B, turbulence suppressing plate 116 is disposed in the vicinity of tongue portion 109 and on an inner peripheral portion side of blades 115 of impeller 103. Turbulence suppressing plate 116 is fixed to suction-side plate 105 of casing 102 and is formed into a shape extending in a radial direction of impeller 103 in a state where turbulence suppressing plate 116 is disposed close to blades 115. Due to the provision of turbulence suppressing plate 116, the direction of a gas flow is converted from tongue portion turbulence 114 generated when a high static pressure is applied to the device to gas flow 117 which is directed to a center portion of impeller 103. That is, with the provision of turbulence suppressing plate 116, it is possible to suppress the inflow of tongue portion turbulence 114 into the pressure boosting start position in the vicinity of tongue portion 109 thus suppressing the retraction of the pressure boosting start position. As a result, sufficient boosting of pressure can be acquired by spiral casing 102 so that performance (static pressure) on a shutoff side can be increased.
In such conventional single suction type centrifugal fan 101, performance (static pressure) on a shutoff side can be increased. However, since turbulence suppressing plate 116 is disposed in a gas flow passage, on an open side on a performance (PQ) curve, that is, in the case where an amount of outflow air is large and at maximum, gas flow noises which occur due to impingement of a gas flow on turbulence suppressing plate 116 are liable to be generated. Further, turbulence suppressing plate 116 and blades 115 of impeller 103 are disposed close to each other and hence, parts are required to satisfy working accuracy and assembly accuracy thus giving rise to a drawback that a working cost and assembling man-hours are increased.
It is an object of the present disclosure to provide a single suction type centrifugal fan which can acquire a high static pressure on a shutoff side on a performance (PQ) curve without additionally providing a part such as a turbulence suppressing plate to a product.